1. Field of the Invention
This invention relates to integrated circuit technology, and more specifically, to a method for performing rework test on integrated circuit (IC) packages, which allows the exposed IC chip contained in the casing of the IC package to be covered up before being mounted on a dedicated test circuit board to undergo function test procedure so as to prevent the inside IC chip from being damaged during the desoldering process, and thereafter allows the cover to be easily and effortlessly detached to allow the test engineer to visually inspect the inside IC chip for any structural problems. This method is particularly useful for performing rework test on IC packages of the ball grid array (BGA) and plastic quad flat package (PQFP) technologies.
2. Description of Related Art
A module IC package integrates one or more IC chips in one single package. Within the IC package, each IC chip is connected to a large number of bonding wires; and therefore, each single IC package typically requires hundreds of wires to interconnect the various IC chips therein. Conventional packaging technologies include, for example, flat package, hermetic and plastic chip carrier, grid array package, quad flat package (QFP), and plastic quad flat package (PQFP), to name just a few.
With conventional IC packaging technology, an IC chip is mounted on a leadframe and then the entire IC chip together with the leadframe are hermetically enclosed in a casing to form a complete IC package. However, with new advances in semiconductor technologies, the functionality of a single IC chip is significantly increased as compared to the earlier types of ICs. This leads to an increase in the total number of bonding wires required to connect the IC chip in the IC package, which would make the packaging more difficult to carry out.
One solution to the foregoing problem is the so-called ball grid array (BGA) technology, which packages an IC chip in a square casing with the tips of the connecting pins being attached with tiny solder balls. When mounting a BGA package on a printed wire board or a printed circuit board, the solder balls are heated to melt, thereby allowing the connecting pins of the BGA package to be conveniently soldered to the bonding pads on the circuit board.
A BGA package is a miniature multilevel printed circuit board on which a plurality of IC chips are mounted. These IC chips are interconnected via printed conductive paths and metal plugs with each other.
In the event that an IC package fails to operate properly after being used for some time, a rework test is customarily performed on the IC package so as to check the functional and structural integrity of the internal circuitry of the IC package. In the rework test, the IC package is typically mounted on a dedicated test circuit board. Moreover, it is usually desired to visually inspect the inside IC chip contained in the casing of the IC package to see if there are any structural defects. To do this, the casing of the IC package should be partly removed to expose the inside IC chip contained in the casing of the IC package.
A conventional way to expose the inside IC chip is to use a high-temperature and high-concentration erosive acid to erode away a selected part of the casing. After this, heated gas and infrared heating means are used together to solder the entire IC package onto the test circuit board. One drawback to this method, however, is that the exposure of the inside IC chip to the heated gas and infrared light can cause the IC chip to be damaged, resulting in open circuits and short circuits in the internal circuitry of the IC chip.
If the IC package is soldered onto the test circuit board prior to exposing the inside IC chip, then the use of the high-temperature erosive acid to remove part of the casing to expose the inside IC chip would easily damage the nearby circuit components on the test circuit board or even the internal wiring in the IC package. One solution to this problem is to position the test circuit board in an upside-down manner and spread the erosive acid upwards onto the casing of the IC package from the bottom side. This scheme, however, is quite difficult to carry out, in that the failure rate is pretty high and the erosive acid can nonetheless easily spread to nearby circuit components on the test circuit board. Therefore, this solution is not quite feasible.
Another solution to the foregoing problem is to cover the opening in the casing with a heat-insulative ceramic cover, and then use adhesive tapes or glues to secure the ceramic cover tightly in position to thereby hermetically seal the inside IC chip from the ambient environment for the purpose of preventing the inside IC chip from exposing to heated gas and infrared light when soldering the IC package onto the test circuit board. This solution, however, is somewhat difficult to carry out, in that, during the soldering process, the heated gas would typically heat the casing of the IC package up to 290.degree. C. to 310.degree. C., and ordinary adhesive tapes or glues would be melted at these temperature levels. The melt can even flow into the inside of the casing and then spread onto the IC chip, thus causing damage to the IC chip. Moreover, the ceramic cover should be air-tightly mounted on the casing of the IC package; otherwise, the heated gas and infrared light can still permeate into the inside of the IC package, causing damage to the inside IC chip.
As a summary, the prior art has the following drawbacks that need to be improved.
(1) First, the scheme of exposing the inside IC chip prior to the mounting of the IC package on the test circuit board would cause the inside IC chip to be easily damaged by the heated gas and infrared light when mounting the IC package onto the test circuit board, resulting in short and open circuits in the internal circuitry of the IC chip.
(2) Second, the scheme of soldering the IC package prior to the selective removal of the casing of the IC package to expose the inside IC chip would easily cause the erosive acid to be spread onto the IC chip, thus causing damage to the IC chip.
(3) Third, in the scheme of air-tightly mounting a ceramic cover by adhesive tapes or glues, the adhesive tapes or glues can be easily melted away when the IC package is heated up to from 290.degree. C. to 310.degree. C., thus breaking up the air-tightness. Moreover, the melt would easily flow into the inside of the IC package and spread onto the IC chip, thus causing damage to the IC chip.
(4) Fourth, in the scheme of using adhesive tapes or glues to secure the ceramic cover in position on the casing of the IC package after exposing the inside IC chip, the same problems described above would also occur, i.e., the adhesive tapes or glues can be easily melted away when the IC package is heated up to from 290.degree. C. to 310.degree. C., thus breaking up the air-tightness, and the melt would then easily flow into the inside of the IC package and spread onto the IC chip, thus causing damage to the IC chip, resulting in short and open circuits in the internal circuit of the IC chip. Moreover, it would make the ceramic cover difficult to be dismounted to allow the test engineer to visually inspect the inside IC chip.